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"What was the price did Father Christmas's sleigh cost? Zero, it was on the house."

This joke is met by moans that resonate through a storage facility in London.

We're at a joke-testing session with a firm that makes products for social events. Its catalogue features festive crackers.

The firm's founder grins, almost apologetically at the joke. But the pun has been selected and will feature in upcoming crackers.

"The success is gauged by the joke by the number of groans and the intensity of the groans at the table," she says.

The key to a good Christmas cracker joke is not the same as a stand-up joke per se. It is entirely about the context - in this case, the shared amusement of the holiday dinner table with elders, kids and potentially neighbours.

"You want the gag to be a thing that brings the eight-year-old together with the 80-year-old," she states.

The Science Behind Communal Laughter

Coming together to enjoy communal laughter is not only nothing new, scientists argue, it is likely to be older than humanity.

"Therefore when you are chuckling with others around the holiday table you are engaging in what's almost certainly a truly ancient mammalian play sound," says a neuroscience expert.

Communal laughter, she explains, aids in make and maintain social connections between individuals.

Researchers have discovered that a lack of such interactions can significantly harm both psychological and bodily well-being.

"The people you talk to, and laugh with, it leads to increased levels of 'happy chemical' release," she adds.

These natural chemicals are the body's "feel-good compounds" and are released both to reduce tension and discomfort and in reaction to pleasurable experiences, such as chuckling with loved ones over a particularly terrible festive cracker joke.

"It's not simply chuckling at a silly joke with a Christmas cracker," she states. "You are actually doing a lot of the truly important task of building, preserving the connections you have with those you care about."

Which Happens In the Brain?

But what is actually happening inside the brain when we hear a gag?

A tremendous amount happens in response to humour, it turns out.

Using functional magnetic resonance imaging (fMRI), a kind of brain scanner which shows which areas of the brain are working harder, researchers have been able to chart the areas that receive more blood.

Testing involves imaging the minds of volunteer subjects and then subjecting them to a collection of humorous words, paired with either a neutral sound, or pre-recorded laughter.

"In the scanner we observed a very interesting pattern of neural activity," says the neuroscientist.

A joke activates not just the areas of the mind in charge of auditory processing and understanding speech, but also neural areas involved in both planning and starting movement and those involved in sight and recall.

Combine all of this as a whole, and people listening to a joke have a complex series of brain reactions that underpin the laughter we hear.

The Contagious Power of Laughter

Researchers discovered that when a funny word is combined with laughter there is a stronger response in the mind than the same word when accompanied by a non-emotional sound.

"This was in areas of the mind that you would employ to move your expression into a smile or a chuckle," she explains.

It means we are not just responding to humorous words, they are reacting to the laughter that follows them.

Amusement, says the professor, can be infectious.

So what does this imply for the laughter found at a Christmas table?

"You laugh harder when you know people," she says, "and laughter increases more when you like them or love them."

When it comes to festive cracker jokes, she explains, the positive factor is more likely to be triggered not by the gag itself, but from the response to it.

"The laughter is key. The gag is the terrible holiday cracker joke, and it's just a reason to laugh as a group."

The Search for the Perfect Festive Pun

Is it possible to find the ultimate gag?

Likely not, but that has not stopped experts from attempting to.

Years ago, a professor set up a research project for the planet's funniest joke.

Over tens of thousands of gags later, with ratings provided by 350,000 participants globally, he has a clearer idea than many as to what succeeds and what fails.

The perfect festive cracker joke must be short, he says.

"They must also need to be poor jokes, jokes that cause us to groan," he adds.

The increasingly "awful" the joke, he states the more effective.

"This is because if no-one laughs – it's the joke's shortcoming, not yours.

"The fascinating part about the holiday cracker jokes is that none of us find them funny.

"That's a common experience at the table and I believe it's lovely."